1. Field of Application
The present application relates to a motor control apparatus incorporating power inverter circuits formed of switching elements.
2. Description of Related Art
Types of motor control apparatus are known which incorporate a DC-to-AC inverter circuit (referred to in the following simply as an inverter circuit) formed of switching elements, for example as described in Japanese patent application publication No. 2009-232620 (designated in the following as reference 1). The inverter circuit of reference 1 is illustrated in the partial circuit diagram of FIG. 3 (based on FIG. 2 of reference 1). The motor control apparatus also includes a smoothing capacitor CN and a control apparatus (not shown in FIG. 3), and is connectable to a DC power source PW, with the positive and negative terminals of the DC power source PW connected to opposing terminals of the smoothing capacitor CN. The inverter circuit is formed of three switching circuits connected in parallel between the terminals of the smoothing capacitor CN. The three switching circuits are respectively formed of a series-connected pair of switching elements SW11, SW12, a series-connected pair of switching elements SW21, SW22, and a series-connected pair of switching elements SW31, SW32, as shown. The junction points of these three series-connected pairs of switching elements are connected to windings of a 3-phase motor M. The control apparatus switches each switching element between an “on” (conducting) state and “off” (non-conducting) state, to convert the DC voltage to a 3-phase AC voltage supply which is applied to the motor M.
After the inverter circuit has commenced operation, a high value of electric charge becomes stored in the smoothing capacitor CN, thereby smoothing the DC voltage from the DC power source PW. Subsequently, when the inverter circuit is disconnected from the DC power source PW, the stored charge in the smoothing capacitor CN remains unchanged. Hence, if the voltage of the DC power source PW is substantially high, there is a danger that the residual charge in the smoothing capacitor CN could cause electric shock to a person. For that reason, when the inverter circuit is disconnected from the DC power source PW, the control apparatus then applies control whereby all of the switching elements of the inverter circuit are set in the on state during an interval of specific duration. That duration is predetermined such that the electric charge in the smoothing capacitor will be sufficiently discharged, while ensuring that excessively high levels of current will not flow in the switching elements. The danger of electric shock can thereby be prevented.
However such a prior art apparatus has the following disadvantage. If there are deviations between the respective timings at which the switching elements of the inverter circuit become switched on, then a momentary high level of current will flow through the windings of the motor M. For example, if the switching elements SW11 (connected to the positive terminal of the smoothing capacitor CN) of one switching circuit, and the switching element SW22 (connected to the negative terminal of the smoothing capacitor CN) of a different switching circuit become switched on concurrently, before the remaining switching elements become switched on, then a momentary high level of current will flow through the motor windings via the switching elements SW11 and SW22. Such an accidental flow of current may cause the motor to momentarily rotate.
Furthermore even without such timing deviations, a similar problem may arise due to variations in device characteristics of the switching elements. For example if the “on” state resistance of a positive-side switching element (connected to the positive terminal of the smoothing capacitor CN) of one of the switching circuits is lower than the “on” state resistance of the positive-side switching elements of the two other switching circuits (e.g., if the “on” resistance of switching element SW11 is lower than that of the switching elements SW21 and SW31), then a current will flow through the motor windings when the charge in the smoothing capacitor is discharged.